1. Field of the Invention
The present invention relates to a field emission cold cathode and a display apparatus using a field emission cold cathode, more particularly to a gate electrode of the field emission cold cathode.
2. Description of the Related Art
A field emission cold cathode is a device which comprises an emitter having a sharp cone-shaped emitter and a gate electrode having an opening of sub-million order, formed close to the emitter, and functions in such manner that it concentrates a high level electric field at a tip of the emitter by the gate electrode, emits electrons from the tip of the emitter under vacuum, and receives the electrons in its anode electrode. In such a field emission cold cathode, discharge of the gate electrode sometimes occurs during operation under vacuum, due to collision of the electrons to the anode electrode and residual gas. The discharge of the gate electrode causes damage such as breaking due to the fusion of materials forming the gate electrode and shorts due to the breakdown of an insulating film under the gate electrode.
In order to prevent such damage due to discharge, various methods have been proposed. By way of example reference is made to a conventional field emission cold cathode disclosed in Japanese Patent Application Laid Open No. 7-240143/1995 is shown in a sectional view of FIG. 1 and a plan view of FIG. 2.
As shown in FIG. 1, a conventional electric field emission cold cathode consists of a silicon substrate 1 serving as a supporting substrate; an insulating film 2 such as an oxide film, formed on the silicon substrate 1; a gate electrode 3a formed on the insulating film 2 and having an opening at an emitter formation region; and an emitter 5a formed in the opening of the insulating film 2, the emitter being connected to the silicon substrate 1; and an insulating film 8 formed so as to cover the gate electrode 3a. An anode electrode 7 is disposed so as to face the gate electrode 3a by spatially separating it from the emitter 5a. As shown in FIG. 1, the gate electrode 3a is of the conventional field emission cold cathode type and has a shape in section such that the side surface of the opening of the gate electrode 3a is approximately perpendicular to the surface of the silicon substrate 1 and the upper surface of the insulating film 8. Moreover, as shown in FIG. 2, when viewed from above, the gate electrode 3a has a configuration which generally includes a rectangular portion having four right angle corners. In such conventional field emission cold cathode, the insulating film surrounds the gate electrode, whereby the occurrence of discharge of the gate electrode due to residual gas near the gate electrode is prevented and the breakdown of the device resulting from a discharge between the emitter and the gate electrode is suppressed.
However, in the foregoing conventional field emission cold cathode, there has been a first problem that a gate voltage required to cause the emitter to emit electrons cannot be reduced. Specifically, since a conventional field emission cold cathode employs a structure in which the gate electrode is surrounded by the insulating film, a margin for depositing the insulating film between the emitter and the gate electrode is necessary, so that operation at low voltage is limited by the amount equivalent to the margin. In order to overcome such problem, an additional mechanism to enhance an electric field must be incorporated into a conventional prior art device, so that complexities of device structure and processes for manufacturing the device result, which entail disadvantages in manufacturing a conventional device.
Furthermore, in a conventional field emission cold cathode, there is a second problem in that breakdown due to discharging from the anode electrode occurs. Specifically, the gate electrode is protected by the insulating film, whereby breakdown due to discharge between the emitter and the gate electrode during operation at low voltage can be prevented effectively. However, the insulating film covering the gate electrode has less effect to prevent the breakdown due to discharge from the anode electrode so that the breakdown of the insulating film under the gate electrode is apt to occur with a high probability.